1. Field of the Invention
The present invention relates to a band compressing/restoring apparatus for performing band compression and band restoration of a television signal.
2. Description of the Related Art
An NTSC system is one of color television broad-casting systems. This NTSC system has compatibility with a black/white television broadcasting system and sufficient performance as a color television broadcasting system as may be seen from its successful operations in, e.g., Japan and the United States.
In a long history of the NTSC system, its image quality has been improved from than that obtained in the initial stage of the system by constant efforts made at both the transmission and reception sides.
In recent years, however, as use of a large-screen display has become widespread, further improvements in the image quality are desired for the NTSC system.
An example of an image quality improving system for the NTSC system is an IDTV (Improved Definition Television) system. In this IDTV system, a transmitted color television signal of the NTSC system (to be referred to as an NTSC signal hereinafter) is perfectly utilized at a reception side to improve the image quality. The IDTV system, which has never been employed with conventional analog processing techniques, is realized by recent progresses in digital processing techniques.
According to the IDTV system, the image quality can be improved from than that obtained by the conventional analog processing.
Since, however, the IDTV system is based on the NTSC system, the upper limit of an improvement in image quality is defined by the standards of the NTSC system.
These limitations are, for examples, as follows:
(1) Aspect ratio of screen; and
(2) Horizontal resolution.
In a current NTSC system, the an aspect ratio in item (1) is 4:3. Therefore, an aspect ratio of the IDTV system is limited to 4:3.
Recently, however, it is confirmed that a ratio of 5:3 or 16:9 is desired by users ("Broadcasting System", edited by Nippon Hoso Kyokai, P. 80). For example, an aspect ratio of 16:9 may eventually be adopted in a high definition television system (CCIR Report 801-2).
In addition, in the current NTSC system, a horizontal resolution in item (2) is 330 Tv. This is because a horizontal transmission band of the current NTSC system is defined to be 4.2 MHz. Therefore, a horizontal resolution of the IDTV system is limited to 330 Tv.
Since, however, the number of effective scanning lines is 480, a current vertical resolution can be set to 450 Tv including a margin such as overscan. Therefore, the horizontal resolution must be increased to obtain a good balance with the vertical resolution.
For this reason, a system capable of improving the above two limitations is desired. In addition, this new broadcasting system is desired to have compatibility with the current NTSC system. This is because the new broadcasting system is assumed to be used together with the current NTSC system for the foreseeable future.
In order to maintain the compatibility with the current NTSC system, a system may be arranged such that a current NTSC signal is used as a main signal and information for an image quality improvement, such as a wide aspect or high precision, is multiplexed as an additional signal.
An example of such a multiplexing system is an SLSC system described in Joseph L. LoCicero, "A Compatible High-Definition Television System (SLSC) with Chrominance and Aspect Ratio Improvements", SWPTE Journal, May 1985.
In this SLSC system, a band of two channels is prepared for one station. That is, a signal basically similar to a current NTSC signal is transmitted through one channel, and the additional signal for an image quality improvement is transmitted through the other.
According to this multiplexing system, the additional signal can be multiplexed without interfering with the main signal.
In this system, however, since two channels are used for one station, channel use efficiency is low. Therefore, in an environment in which channel assignment has almost reached its limit, as in Japan, it is difficult to carry out this system. In addition, in order to perform intra- or inter-station transmission, all of the television broadcasting apparatuses must be replaced with new ones since no current television broadcasting apparatus has a band reaching 10 MHz which as is defined by the SLSC system.
For this reason, a system in which an additional signal can be multiplexed within a band of one channel is desired. In addition, this multiplex operation is desired to be performed within 4.2 MHz as a base band in order to maintain the compatibility with current television broadcasting apparatuses such as a video tape recorder and a transmitter.
An example of a system capable of multiplexing an additional signal within 4.2 MHz as a base band is described in T. Fukinuki et., "Extended Definition TV Fully Compatible with Existing Standards", IEEE Tr. on Communication, Vol. COM-32, No. 8, August 1984.
In this multiplexing system, a spectral region which is not used in the case of a still image is used to multiplex an additional signal (a brightness detail component having a horizontal band of about 4 to 6 MHz) for increasing the horizontal resolution of an NTSC signal. In this case, predetermined regions of the first and third quadrants in two-dimensional spectral display in vertical and time directions are used as a multiplexing region.
According to this multiplexing system, the horizontal resolution of a still image can be increased.
This system, however, cannot increase the horizontal resolution of a motion image. This is because a main signal and an addition signal of a motion image cannot be separated from each other at a reception side since a spectrum of the motion image is expanded in the time direction. Therefore, this system cannot be applied to multiplexing of an additional signal for obtaining a wide aspect. That is, since an improvement in horizontal resolution is effective especially for a still image, no problem is posed although the system can multiplex an additional signal of only a still image. An additional signal for obtaining a wide aspect, however, is required for both still and motion images. Therefore, this multiplexing system capable of transmitting additional information for only a still image cannot be adopted since a wide aspect cannot be obtained for a motion image.
For this reason, a demand has arisen for a system capable of multiplexing an additional signal within a base band for not only a still image but also a motion image.
An example of such a multiplexing system is described in, e.g., M.A. Isnardi, et., "Encoding for Compatibility in the ACTV System", IEEE Trans. on Broadcasting Vol. BC-33, No. 4, 1987, PP. 116 to 123.
In this ACTV system, band compression is performed for main and additional signals to enable multiplexing of the additional signal for both still and motion images.
In the ACTV system, however, a band in the vertical and time directions of each of the main and additional signals is largely limited. Therefore, the motion of a reproduced image becomes unnatural, or its vertical resolution is degraded.
These problems will be described below by taking a main signal as an example.
In the ACTV system, an in-frame average of components higher than 1.5 MHz (horizontal frequency) of the main signal is calculated and output by field-repetition.
Therefore, in the case of a still image (i.e., when a time frequency is 0 Hz), a vertical upper-limit frequency of components higher than 1.5 MHz (horizontal frequency) of the main signal is limited to 525/4 [cph] which is half that of the NTSC system. Therefore, in the case of a still image, a vertical resolution of the components higher than 1.5 MHz, i.e., an oblique resolution is degraded. As a result, a blurred still image is obtained.
In the case of a motion image, as in the case of a still image, a vertical band is limited to half that of the NTSC system. Therefore, a vertical frequency becomes 0 [cph] at a portion in which a time frequency is 15 Hz. As a result, since only motion components up to 15 Hz can be transmitted, smoothness of the motion is largely degraded.
As described above, the ACTV system has a problem of unnaturalness of a motion or a decrease in vertical resolution.
Attention must be paid to this ACTV system, however, since the system can multiplex an additional signal of even a motion image by adopting a band compressing technique. Therefore, a demand has arisen for a band compressing technique not causing the unnaturalness of a motion or a decrease in vertical resolution as a drawback of the ACTV system.
In addition, when an additional signal is to be multiplexed with a main signal, a visual interference of the additional signal with respect to the main signal poses another problem. Therefore, a band-compressed output of an additional signal is desired not to visually interfere with a main signal.